The present invention relates to a display device having a transparent cover, which serves as a sound source, made of a transparent body to be arranged in front of a display panel of the display device.
In a prior art wrist watch having an alarm function, a buzzer is mounted in a back rid on a back of a case so that sound is generated by vibrating the buzzer and propagated to the exterior through sound holes formed in the back rid. In such a type of wrist watch, however, if the sound holes in the back rid are eliminated the sound propagated to the exterior is attenuated, while a water proof structure is not attained because of the sound holes. In addition, because of the structure to protect the vibrating reed, the thickness of the watch increases.
In a recently proposed wrist watch as shown in FIG. 1, a transparent cover 5 to be mounted in front of a display panel 1 of a body 4 which contains the display panel 1, a drive source 2 and a watch driver 3 is directly vibrated so that the cover 5 functions as a sound generator. As shown in FIG. 2, in order to vibrate the cover 5, a piezoelectric vibrator 10 which is made of a transparent piezoelectric ceramic plate and has transparent electrodes 8 and 9 arranged on both surfaces thereof is bonded by transparent adhesive material 7 on an inner surface of the cover 5 having a transparent electrode 6 coated thereon. A voltage is applied to the transparent piezoelectric vibrator 10 through a contact 11 which contacts the transparent electrode 6 of the cover 5 and an electric contact 12 which contacts the exposed one 9 of the transparent electrodes of the transparent piezoelectric vibrator 10 so that the integrated cover 5 vibrates to generate sound. The cover 5 is coupled to a body 4 through a waterproof and anti-shock packing 13 which functions as a washer.
With respect to the piezoelectric element to be used in the wrist watch of such a structure which utilizes the transparent cover as the sound generator, there are various problems to be further resolved. They are:
1. Liquid crystal used as the display element looks poor in contrast ratio when it is viewed obliquely.
2. A high quality of display, sufficient volume of sound and ability to protect against a mechanical shock are not attained and the cost increases unless relative dimensions of and positional relation of the piezoelectric element and the transparent cover are properly defined.
It is an object of the present invention to provide a display device which overcomes the problems based on the piezoelectric vibration element.
According to one aspect of the present invention, there is provided a display device having a transparent cover serving as a vibrator of a sound generator, said transparent cover having a transparent piezoelectric element coated with transparent electrodes on both surfaces thereof, bonded to an inner surface of said transparent cover by transparent adhesive material, said transparent cover being arranged in front of a display panel of said display device, wherein said transparent electrodes are used as anti-reflection films for the light near ultraviolet ray.
According to the second aspect of the present invention, there is provided a display device having a transparent cover serving as a vibrator of a sound generator, said transparent cover having a transparent piezoelectric element coated with transparent electrodes on both surfaces thereof, bonded to an inner surface of said transparent cover with transparent adhesive material, said transparent cover being arranged in front of a display panel of said display device, wherein the length l and the thickness t.sub.1 of said transparent cover and the thickness t.sub.2 of said piezoelectric element are selected to meet the following relations: EQU 5.6.times.10.sup.2 (t.sub.1 +t.sub.2).ltoreq.l.sup.2 (a) EQU 2.5.times.10.sup.3 (t.sub.1 +t.sub.2).sup.3 .ltoreq.l.sup.2 (b) EQU t.sub.2 .gtoreq.0.1 mm. (c)
According to the third aspect of the present invention, there is provided a display device having a transparent cover serving as a vibrator of a sound generator, said transparent cover coated with a transparent piezolectric element coated with transparent electrodes on both surfaces thereof, bonded to an inner surface of said transparent cover with transparent adhesive material, said transparent cover being arranged in front of a display panel of said display device, wherein an area of the transparent cover other than a specific view area is formed as a shielding area, said piezoelectric element extends to cover mid-points between the center and the periphery of said transparent cover, and the size of said piezoelectric element is determined to be over such a minimum size that the periphery thereof is not placed inside of said view area.
According to the fourth aspect of the present invention, there is provided a method for manufacturing a piezoelectric element body to be bonded to a transparent cover of a display device after slicing a cylindrical transparent piezoelectric ceramic boule into disks and polishing them, and then cutting away predetermined peripheral areas of the polished disks to form the desired shape wherein when a single piezoelectric element body is to be cut out from one of the said sliced disks, the ingot diameter is selected to be 0.7 to 0.9 times as large as the outer diameter D.sub.1 of said transparent cover, and when double piezoelectric element bodies are to be cut out from one of said sliced disks, said ingot diameter is selected to be 1/0.7-1/0.9 times as large as said outer diameter D.sub.1, and said sliced bodies are cut such that the outer periphery of said ingot remains at least two corners of said piezoelectric element body.